Question

Alright, I’m trying to link three different tables (Customers, Orders, and Order Details), in an effort to figure out which customers ordered the same item twice. I’m stumped. Here’s what I’ve come up with thus far:

Select ContactName, Orders.OrderID, ProductID From Customers
inner join Orders on Customers.CustomerID = Orders.CustomerID
join [Order Details] on [Order Details].OrderID = Orders.OrderID;

I was thinking it’d be easiest to find ProductIDs that have the correlating ContactName but a different Orders.OrderID… and this is what I’m having trouble with. Any help would be much appreciated.

Answer 1

you need to select two different orders which have a detail with the same product. (“select all customers which have two different orders with the same product”)

Select 
  ContactName, 
  o1.OrderID, 
  o2.OrderID, 
  od1.ProductID 
From 
  Customers
  inner join Orders o1 on Customers.CustomerID = o1.CustomerID
  join [Order Details] od1 on od.OrderID = o1.OrderID;
  inner join Orders o2 on Customers.CustomerID = o2.CustomerID
  join [Order Details] od2 on od.OrderID = o2.OrderID;
where 
  o1.orderid <> o2.orderid
  and od1.productid = od2.productid

or use a subquery. (“select all customers which have a product detail with the same product as in another order of the same customer”);

Select 
  ContactName, 
  o1.OrderID, 
  od1.ProductID 
From 
  Customers
  inner join Orders o1 on Customers.CustomerID = o1.CustomerID
  join [Order Details] od1 on od.OrderID = o1.OrderID;
where 
  od1.productid IN
    (SELECT od2.productid 
    FROM
      Orders o2 on Customers.CustomerID = o2.CustomerID
      join [Order Details] od2 on od.OrderID = o2.OrderID
    where 
      o1.orderid <> o2.orderid)

Or use group by. then you can count them. You can aggregate different values, for instance the total amount ordered or the total prize payed.

Select 
  ContactName, 
  od1.ProductID
  count(*) as "number of repeated orders details",
  count(distinct o1.orderid) as "number of different orders"
From 
  Customers
  inner join Orders o1 on Customers.CustomerID = o1.CustomerID
  inner join [Order Details] od1 on od.OrderID = o1.OrderID;
group by 
  ContactName, 
  od1.productId
having 
  count(distinct o1.orderid) > 1